NovikovO. A., and RukasovV. I. 1997. “Approximation of the Classes $C_{β}^{ψ} H_{ω}$ by Generalized De La Valiée-Poussin Sums”. Ukrains’kyi Matematychnyi Zhurnal 49 (4), 606-10. https://umj.imath.kiev.ua/index.php/umj/article/view/5039.